Top-level category page to show all categories

i would plug the category rewrite rules something like this

<?php
/*
Plugin Name: CategoryOverview
Plugin URI: http://rathercurious.net
Description: redirects a blank category name to an overview page
Author: Justin Adie
Version: 0.1
Author URI: http://rathercurious.net
*/

add_filter('category_rewrite_rules', array('categoryOverview', 'rewrite'));

class categoryOverview{
	public function rewrite($data){
		$catBase = get_option('category_base');
		$array[$catBase.'$'] = 'index.php?category_name=overview';
		return array_merge($data, $array); //use array_merge to force this one on to the end so it gets matched last
	}
}
?>

then you would create a category called overview, grab the id associated with it and create a template file called category_[id] and this would be used to display your overview.
if you want, you could suppress the overview category from the various places that categories are displayed too, although in reality if you’re the only person posting this isn’t a real issue.

of course, you can also redirect to any other page that you want too: just change the array element value. or use the template_redirect hook.

actually we ended up with a neater solution than a permalink so do check out BRPS’ link above for the full discussion and solution.